The requirements for audio power amplifiers are increasingly becoming more and more demanding. This is especially true in the field of mobile audio power amplifiers, where recent advances in consumer technology have delivered very high quality signal sources and output transducers to the general populous.
In response to these conditions in the market place, astute designers have adopted the power Vertical field effect transistor (VFET) as the output device of choice for bilateral class AB linear power amplifiers. This choice stems mainly from the recognition of the power VFET's superiority in terms of ruggedness, linearity, speed and drive requirements relative to that of other power device technologies. Interestingly, most modern day power VFET amplifiers that have been devised utilize complementary VFET output devices (P & N) in the power output stage. The one significant short coming of this configuration is the cost and performance penalties of the P type VFET. For example, the P type VFET usually costs about twice that of the N type VFET and delivers only about 70% of the gain, usually with a decrease in speed. This unfortunate situation has led designers to pursue amplifiers where all of the output devices are N-channel VFET's. Several designs have been conceived and reduced to practice but they are plagued by problems in three key areas. The problem areas include: (1) lack of stable output stage bias compensation with simple circuitry that does not impair rail to rail output swing under load; (2) excessive common mode conduction in the output stage VFET's; and (3) unsatisfactory high frequency distortion characteristics with simple circuitry (no more than 3 active devices per stage, 2 serial stages maximum).
The first problem area, lack of stable output stage bias compensation, has been solved by placing a sensing device with a negative temperature coefficient in proximity to the output devices (i.e. the heat sink) and having the sensing device control the output stage bias via thermal feedback. Previous embodiments of this technique in N type VFET output stage amplifiers have required the control of the sensor to be effected or "programmed" from two or three stages away from the control "target" (i.e. gate to source junction of the VFET) in an open loop manner, consequently, excessive and unsatisfactory drift of the bias point occurs.
Temperature independent biasing of the output stage has also been achieved via all electronic techniques (i.e. no thermal feedback). Electrically controlled output stage idle current is achieved by actively monitoring the output stage bias current and comparing it to a predetermined reference via electronic negative feedback. The shortcoming of this approach has always been one of separating the class A idle currents (e.g. 50 ma) from the large class B peak currents (e.g. 30 A) that flow through the sense point node. A few designs have managed to pull this off, however, the complexity of this resultant "Autobias" circuitry strongly offsets any real gain in almost any conceivable circumstances.
The common mode problem has been solved in the past by reducing the voltage swing of the output stage. However, in reduced load impedance applications, reduced voltage swing causes unacceptable power loss in the output stage. The source of the problem stems from the input capacitance of the VFET. Adequate charging and discharging currents must be made available at the highest operating frequency if common mode output stage currents are to be avoided at full rail to rail output swing. The prior art has not allowed for this in a simple non-parts intensive way.
The distortion problem has been solved via application of additional stages of gain to increase the correction power of the loop. This has achieved a certain measure of success, however, a substantial penalty has been paid in terms of increased circuit complexity.